Hybrid Fiber Coax (HFC) networks have been used for some time to implement or upgrade metropolitan area cable television systems. In a recent improvement to HFC networks, mini-Fiber-Nodes (mFNs) have been employed to extend fiber closer to subscribers, each mFN providing bidirectional fiber-based services (including Internet access via cable modem) for multiple-tens of subscribers. mFNs are intended to be compact units deployed in the field, generally in aerial-wire or utility-pole-mount configurations, and include at least one “upstream” receiver. An upstream receiver is a receiver in the return direction, that is, in the direction from the subscriber to the cable head end. Generally multiple coaxial (coax) cables, each carrying upstream signals corresponding to a different group of subscribers, may terminate at the mFN for subsequent upstream transmission via fiber.
Each of these coax cables generally has a wide spectrum (up to 1 GHz) including a multiplex of upstream carriers located in the 5-42 MHz range. In accordance with the DOCSIS industry standard, each upstream carrier may have any symbol rate from 5 possible values (160 kBaud, 320 kBaud, 640 kBaud, 1280 kBaud, 2560 kBaud) with a modulation format of either QPSK or 16-QAM. In addition, each upstream carrier has a power spectrum density variation, with respect to a nominal value, of ±6 dB. In a worst-case scenario for the upstream receiver, the 5-42 MHz band may be filled with (up to 11) undesired carriers operating with the maximum baud rate (2560 kBaud) and a desired carrier may have the smallest value 160 kBaud. Furthermore, there may be up to a 12 dB difference in power spectrum density between the desired carrier and the others.
A digital receiver may be partitioned into a tuner and digital demodulator portions. The tuner accepts one or more broadband analog inputs having a variety of desired and undesired signals. The tuner's purpose is to isolate the desired signals and provide baseband-translated digitized equivalents for subsequent processing in the demodulator. Because quadrature modulation is common, the outputs are often provided in quadrature. The stage(s) where frequency translation is performed is (are) generally referred to as the receiver front-end. The stages where digitization is performed may be referred to as the A/D (ADC, analog to digital converter, or digitizer). The stages before and after the A/D are necessarily analog and digital, respectively.
A traditional tuner for an upstream digital receiver is shown in FIG. 1. A first IF (intermediate frequency) conversion combined with a SAW (surface acoustic wave) filter is used for isolating the desired carrier and suppressing any and all undesired carriers. Next, down-conversion is done in the analog domain by in-phase splitting the isolated carrier and mixing with 2 analog LOs (local oscillators). The analog LOs are provided in quadrature at the expected channel spacing. Each of the resulting quadrature signals from the mixers is then coarse anti-alias filtered in the analog domain and sampled with respective A/Ds. Matched filtering after the A/Ds is subsequently performed in the digital domain. The quadrature output signal pair of the matched filter may then be provided to the demodulator of the desired carrier/channel. There are a number of significant costs associated with each required stage of any digital receiver tuner. Costs are minimized by keeping stages as simple as possible and as few in number as necessary. Another stage cost is associated with the bit-width of the digital stages. Each additional bit in width incrementally increases the cost, size, and complexity of the associated receiver. Since the dynamic range of the signals being processed necessitates a proportional bit-width, the dynamic range should be minimized consistent with maintaining good performance. Because A/Ds are often the most complex and expensive sub-systems in the tuner, the number of required A/Ds is a key implementation consideration. In addition, each A/D has an associated clock sub-system (not shown in FIG. 1 but known to practitioners of the art) that must be configured appropriately for the carrier being digitized. The number and extent of required clock sub-systems is thus also an important implementation consideration.
Traditional digital receiver approaches have required the entirety of the above-described tuner per desired carrier/channel. In the mFN augmented HFC systems discussed above, because of the associated expense, bulk, and complexity of the traditional tuners, it has not been considered practical in widespread deployment to demodulate upstream signals locally at the mFN. Accordingly, the entire spectrum of each coax has been indiscriminately sent upstream over fiber via either analog or digital techniques for remote reconstruction and demodulation at the cable head end (or intermediate upstream location). Accordingly, when multiple coaxial cables terminate into the mfN, ether multiple expensive fibers are required or expensive WDM techniques have been used to multiplex the multiple coax spectra onto respective “lambda” wavelengths of a single fiber. In these approaches, the ultimately demodulated upstream information content is a small fraction of the transmitted upstream bandwidth.
In applications (such as upstream cable modem traffic) where there are multiple desired channels on each of generally multiple input coaxial cables, the use of traditional digital receivers (requiring one tuner per desired carrier/channel) results in a confusing proliferation of associated splitters, connectors, and couplers. Provisioning (establishing the operating configuration of) a particular upstream subscriber signal (out of many) over a particular upstream channel (again out of many) generally necessitates error-prone manual configuration of multiple coaxial cables, splitters, connectors, and couplers. This hardware also introduces new noise and signal losses. Provisioning additionally involves configuration of the receiver to accommodate the carrier frequency and baud-rate of the transmitted signal. To change the carrier frequency dynamically in the traditional digital receiver requires a very agile and very costly local oscillator. Less expensive, less agile, local oscillators generally necessitate error-prone manual configuration of component modules or component settings.
It is thus seen that there are many shortcomings to the traditional tuner approaches to multiple input, multiple output channel, digital receivers. What is needed is an optimized tuner for such receivers that is efficient, cost sensitive, flexible, and minimizes noise and signal loss. What is needed is a multiple input, multiple output channel, digital receiver tuner that reduces the number of signal processing stages, the stage bit-widths, the number of A/Ds, and the number and extent of clock sub-systems compared with traditional approaches. What is further needed is a compact and efficient multiple input, multiple output channel, digital receiver that performs local demodulation and is suitable for widespread field deployment in distributed communication systems and networks.